Ah, if she hasn't I'd have said could it be a possible on the way which could account for blood showing up.
Did it just show up on a dipstick at the doctors or did they send it to the lab for analysis and did they say what kind of blood cells they found and levels?
If low levels then things can come and ok hence the benefits of retesting. I had a run of things showing up, blood, protein, leukocytes and things on blood tests too but often they would have disappeared on another test. I didn't have stinging either and had some treatments for utis then swabs done which found a high level of yeast so treated for that - my GP said that could cause uti like symptoms.
As I was back and forth doing wee tests, I got myself some of those self test strip things so I can test my own so that if nothing showed up I would not bother my GP (and vice versa as appropriate) especially as I have to ring her enough anyway with other things. I see traces of things appear and disappear from test to test (not that I do it regularly) and it's been quite revealing and I wouldn't bother my doctor unless specific things showed up pretty high or kept showing up or I had other symptoms.
Good they are keeping an eye on things and, if trace amounts, I wouldn't worry too much.